The invention relates to time-domain equalization in a discrete multi-tone (DMT) receiver.
Conventional single carrier modulation techniques translate data bits for transmission through a communication channel by varying the amplitude and/or phase of a single sinusoidal carrier. By contrast, DMT, which is also referred to as Orthogonal Frequency Division Multiplexing (OFDM) or Multicarrier Modulation (MCM), employs a large number of sinusoidal subcarriers, e.g., 128 or 256 subcarriers. The available bandwidth of the communication channel is divided into subchannels and each subchannel communicates a part of the data. A DMT system may employ quadrature amplitude modulation (QAM) for each of the subcarriers.
OFDM-based systems transmit blocks of information bits. The time required to transmit one such block is called the symbol period. The time domain waveform that corresponds to one such block of bits is called a symbol.
Intersymbol interference (ISI) arises from the characteristics of practical communication channels and limits the rate at which information can be transmitted through them. Specifically, communication channels typically have an Effective Discrete-Time Impulse Response (EDIR) that is greater than one sample time in length, which causes ISI. ISI is a well-known phenomenon in single-carrier communication systems and there are many techniques for reducing it. The process of such ISI reduction is called equalization. ISI is discussed, for example, in Proakis, Digital Communications, McGraw Hill, 2nd Edition, 1989.
Equalization in OFDM-based systems is achieved by a two stage process. First, at the transmitter, an end-portion of each symbol is affixed to the beginning of the symbol to form what is called a Cyclic Prefix (CP). A cyclic prefix that is greater than the EDIR of the channel prevents one symbol from interfering with another. Furthermore, it also facilitates a simple method of neutralizing the time-domain spread of each symbol forced by the channel. This is achieved by the use of a simple frequency domain process in the receiver which requires one multiplication operation for each used subcarrier of the OFDM system.
The use of a Cyclic Prefix to reduce ISI is discussed, for example, in: Cimini, xe2x80x9cAnalysis and Simulation of a Digital Mobile Channel using Orthogonal Frequency Division Multiplexing,xe2x80x9d IEEE Transactions on communications, pp 665-675 July 1985; Chow, xe2x80x9cA Discrete Multi-Tone Transceiver System for HDSL applications,xe2x80x9d IEEE Journal on Selected Areas of Communications, 9(6):895-908, August 1991; xe2x80x9cDMT Group VDSL PMD Draft Standard Proposal,xe2x80x9d Technical Report, T1E1.4/96-329R2, ANSI 1997.
Another problem arising in conventional DMT systems is noise bleeding, which is the phenomenon of noise in one frequency band interfering with a signal whose subcarrier is in another frequency band. Noise bleeding is caused, in general, by the Discrete Fourier Transform (DFT) operation at the receiver. Noise bleeding is discussed in, for example, Worthen et. al., xe2x80x9cSimulation of VDSL Test Loops,xe2x80x9d Technical Report T1E1.4/97-288, ANSI 1997.
In a perfectly synchronized DMT system, the signal in one frequency band does not interfere with a signal whose subcarrier is in another frequency band. However, noise from one band may interfere with other less noisy bands and render them unusable. Techniques for dealing with noise-bleeding include wavelet-based solutions. However, wavelet-based solutions are, in general, computationally intensive.
The invention provides a spectrally constrained impulse shortening filter (SCISF) for use, for example, in DMT systems. The SCISF serves two primary functions.
First, the SCISF reduces intersymbol interference (ISI) by reducing the length of the effective discrete-time impulse response (EDIR) of the communication channel. Conventional impulse shortening filters may have deep nulls in their frequency response. By contrast, the SCISF has a filter characteristic that is essentially free from undesired nulls that may attenuate or completely eliminate certain subcarriers.
Second, the SCISF reduces noise bleeding between subchannels by attenuating noisy channels in a manner that does not reduce the signal to noise ratio (SNR) in these channels, but reduces the noise power that may appear in the sidelobes of adjacent subchannels. The SCISF accomplishes these functions by applying a frequency constraint to the signal based on a target spectral response.
In one general aspect, the invention features equalizing a channel in a multiple carrier communication system. The system includes a spectrally constrained impulse shortening filter. Received noise power spectral density is measured. A target spectral response is computed based on the measured noise power. A frequency response for the spectrally constrained impulse shortening filter is selected based on the target spectral response. The communication signal is filtered with the spectrally constrained impulse shortening filter.
Embodiments may include one or more of the following features. The noise power spectral density may be measured at the output of the discrete Fourier transform. The spectrally constrained impulse shortening filter may be a time domain digital filter.
In another aspect, the invention features equalizing a channel in a multiple carrier communication system. The channel has an impulse response and is configured to receive a signal having a cyclic prefix. A target spectral response is computed. The impulse response of the channel is shortened so that a significant part of the energy of the impulse response is confined to a region that is shorter than a target length, and the signal is filtered based on the target spectral response.
The target length may be a length of the cyclic prefix. The target spectral response may be computed from measured noise power density, which may be measured at the output of a discrete Fourier transform. For example, the target spectral response may be the inverse of the measured noise power spectral density. The filtering step may be performed with a filter having a frequency response selected to match the target spectral response. The shortening of the impulse response and/or the filtering may be performed by a time domain digital filter.
In another general aspect, the invention features selecting an impulse response for a spectrally constrained impulse shortening filter in a multiple carrier communication system. Received noise power spectral density is measured. A cost function is computed using the noise power. The cost function is dependent on the impulse response. The dimensionality of a space over which the cost function is defined is reduced and the cost function is minimized. The noise power spectral density may be measured at the output of a discrete Fourier transform. The cost function may be used to compute coefficients for the spectrally constrained impulse shortening filter.
In another general aspect, the invention features a spectrally constrained impulse shortening filter for a multiple carrier communication system. The system includes a channel that has an impulse response. A digital filter structure receives the signal and apply a frequency characteristic to the signal. The frequency characteristic is determined by filter coefficients. Filter coefficients are selected to shorten the impulse response of the channel so that a significant part of the energy of the impulse response is confined to a region that is shorter than a target length and to apply a frequency characteristic to the signal based on a target spectral response. The selected filter coefficients are input to the taps of the filter.
In another general aspect, the invention features a receiver for receiving a multiple carrier signal from a communication channel having an impulse response. An analog-to-digital converter receives the signal from the communication channel. A spectrally constrained impulse shortening filter receives the signal from the analog-to-digital converter and shortens the impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a target length. The filter also applies a frequency characteristic to the signal based on a target spectral response. A discrete Fourier transform receives the output of the spectrally constrained impulse shortening filter and a decoder receives outputs of the discrete Fourier transform.
In another aspect, the invention features a modem in which an encoder receives digital data and outputs a constellation of complex values. An inverse discrete Fourier transform receives the constellation from the encoder. A digital-to-analog converter is connected to the inverse discrete Fourier transform and outputs a signal to a communication channel. An analog-to-digital converter receives the signal from the communication channel. A spectrally constrained impulse shortening filter shortens an impulse response of the channel so that a significant part of an energy of the impulse response is confined to a region that is shorter than a target length. The signal is filtered based on a target spectral response. A discrete Fourier transform connected to the filter and a decoder is connected to the discrete Fourier transform and outputs digital data.
In another aspect, the invention features software for causing a processor in a communication system to perform the following operations: measure received noise power spectral density; and compute a target spectral response based on the measured noise power. The software may also include instructions for causing a processor to compute filter coefficients based on the target spectral response.
In another aspect, the invention features software for causing a processor in a communication system to measure received noise power spectral density. A cost function is computed using the noise power. The cost function is dependent on an impulse response of a spectrally constrained impulse shortening filter. The dimensionality of a space over which the cost function is defined is reduced, and the cost function is minimized.
The techniques described here are not limited to any particular hardware or software configuration. They may find applicability in any computing or processing environment that may be used for a communication system. The techniques may be implemented in hardware or software, or a combination of the two. Preferably, the techniques are implemented in computer programs executing on a digital signal processor that includes a processor and a storage medium readable by the processor (including volatile and non-volatile memory).
Other features and advantages will be apparent from the following description, including the drawings and the claims.